Prostate cancer is the most common malignancy for older men and is a major cause of death for that population. Until recently, it was believed that reduction of testosterone was a key component in treating patients diagnosed with prostate cancer. However, a large number of patients having prostate cancer do not respond to reduction of testosterone levels instigated by luteinizing hormone releasing hormone (LHRH) agonists and were thereby dubbed as having “hormone resistant” cancer. Only half of these patients having “hormone-resistant” prostate cancer respond to hormonal treatments.
It is currently recognized that LHRH agonists or antagonists do not completely reduce circulating testosterone levels due to sources other than the testes that can synthesize testosterone, including the adrenal gland and the prostate tumors themselves. The cytochrome P450 (CYP) enzymes include a large family of highly conserved enzymes, including CYP17, that are involved in the synthesis of cholesterol and other bioactive steroids. The fact that these enzymes are involved in steroid hormone biosynthesis has led to recent findings that castration-resistant prostate cancer in men and certain breast cancers in women are responsive to CYP17 inhibition.
CYP17 is a key enzyme in the production of androgenic steroids in many tissues, including prostate tumors, and catalyzes the 17α-hydroxylase reaction and 17,20-lyase reaction of both progesterone and pregnenolone. Inhibition of CYP17 results in reducing the levels of dehydroepiandrostenedione (DHEA) and androstenedione, which are weak androgens and precursors that are subsequently converted to testosterone and dihydrotestosterone by other enzymes.
Designing inhibitors of CYP17 is problematic for several reasons. First, there is limited information regarding the structure of this enzyme. Second, human CYP17 is not available from natural sources, thereby requiring its recombinant generation. Ketoconazole has been used to inhibit CYP17, but is not very potent and is non-selective since it inhibits other CYP enzymes. Other CYP17 inhibitors have been reported, and the steroidal CYP17 inhibitor Zytiga™ (abiraterone acetate) was recently approved by the U.S. Food and Drug Administration (FDA) for use in combination with prednisone for the treatment of patients with metastatic castration-resistant prostate cancer (CRPC) who have received prior chemotherapy containing docetaxel. Most CYP17 inhibitors, however, including both steroidal compounds such as abiraterone and non-steroidal compounds, have limited selectivity for CYP17, short in vivo half-lives, and/or poor bioavailability.
What is needed are alternative medications for treating prostate and other cancers that function by inhibiting CYP17.